The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the accurate measurement of nutation angle, or flip angle, prior to a scan by an MRI system.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The degree to which the net aligned moment is tipped into the x-y plane is determined by the magnitude of the applied excitation field. The desired "nutation" angle depends on the specific imaging pulse sequence used. For spin echo scans which use rf refocusing pulses, it is common to nutate the nuclei by 90.degree. to get the highest possible signal. For gradient echo scans, it is common to nutate the nuclei by a smaller angle, for example 30.degree. in order to disturb the equilibrium as little as possible. The nutation angle actually resulting from the applied rf magnetic field depends on many factors including the rf power applied to the coil, the shape of the rf magnetic field envelope and the patient shape and proximity to the coil. It is difficult to predict exactly what rf power is needed for the desired nutation angle. Therefore it is common practice to measure the rf power needed in a calibration step performed prior to the MR imaging scan.
Prevailing prescan methods for measuring nutation angle, such as that described in U.S. Pat. No. 5,107,215, acquire a set of 10 to 15 NMR echo signals produced with different rf power settings. The power setting providing the largest signal is assumed to produce a 90.degree. nutation angle, and this power setting may be scaled to produce other nutation angles. Such measurements require 20 to 30 seconds to perform, which is not long when employed with spin echo scans in excess of one minute.
Most MRI scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. When used with these fast pulse sequences, prevailing prescan methods requiring 20 to 30 seconds are unsatisfactory.
Faster prescan methods, such as those described in U.S. Pat. Nos. 4,814,708 and 4,983,921 have been proposed. These methods use three rf pulses, all of the same nutation angle, causing four separate echoes which are frequency encoded in the slice-select direction. The echoes may be processed in various ways to give estimates of the flip angle resulting from the applied rf power. The rf power needed for any other nutation angle is obtained by scaling the measured value.
The fast methods have several problems. The rf magnetic field varies spatially within the patient, and if the rf power is set so that nuclei are nutated by 90.degree. at a location where the rf field is low, the nutation angle will be significantly greater than this at locations where the rf field is large. The result is noticeable shading in the image. In order to overcome this problem it is necessary to measure the variation of the nutation angle within the imaging plane. Since the fast methods measure the nutation angle orthogonal to this plane, the measured nutation angle corresponds to an average over the imaging plane and may be significantly smaller than the maximum nutation angle within this plane. This will result in the rf power being chosen too high with subsequent shading in the image. A second problem is that prior fast methods have substantially worse SNR than the current slow method. The current slow method has good SNR because the result is obtained by a fit of data from many acquired NMR signals, whereas the fast methods employ two to four NMR signals obtained during a single measurement pulse sequence. A third problem with some of the fast methods is that their result depends strongly on the T.sub.1 of the subject. As the T.sub.1 becomes shorter, the estimate of nutation angle decreases, causing the rf power used to be correspondingly higher with subsequent shading in the image.